The invention relates to fiber-optic sensors, in particular fiber-optic gyroscopes and fiber-optic sensors that measure magnetic fields.
Fiber-optic sensors can be used to measure various physical quantities, such as the rotation of an object (referred to as fiber-optic gyroscope) or magnetic fields arising from currents in the vicinity of the sensor (referred to as fiber-optic current sensor). Such fiber-optic sensors typically include a section of optical fiber that is coupled to a broadband source, such as a laser operated below threshold, a superluminescent diode (SLD) or a fiber superluminescent source, a coupler to couple optical radiation emitted by the broadband source into the fiber, preferably a fiber coil, at least one polarizer and at least one phase modulator, and a detector which detects a phase shift or polarization difference between the optical signals traversing the fiber. The phase or polarization shift between the optical signals may be introduced, for example, by the rotation of the fiber coil or by a magnetic field. The terms fiber-optic sensor and gyro will be used interchangeably hereinafter to refer to both fiber-optic gyroscopes and fiber-optic current sensors.
The electrical output signal of a fiber-optic sensor includes noise components arising from various sources. The output noise which tends to manifest itself as angular random walk (ARW), as defined, for example, in IEEE Std-528, Inertial Sensor Terminology (incorporated by reference herein), has a different functional dependence on the optical power at the photodetector input, depending on the origin of the noise component. For example, thermal noise generated in the transimpedance amplifier feedback resistor in the photodetector electronics is independent of the light power. Shot noise related to the quantized nature of the detector current can be shown to be proportional the square root of optical power, whereas relative intensity noise (RIN), which is inherent in the light of the source due to its finite bandwidth and impinges on the photodetector and flicker noise (1/f), is proportional to the optical power. Since RIN and flicker noise have the same functional dependence on optical power, the term RIN hereinafter refers to both RIN and flicker noise.
The RIN causes the noise-related performance of the fiber-optic sensor systems to saturate, rather than continue to improve, as the source power is increased. Unless the RIN can be mitigated, there exists a power level beyond which no further practical improvement is possible. For example, EDFS (Erbium doped fiber sources) which, due to their high optical power and wavelength stability, are often considered the natural choice for high performance fiber-optic sensors, tend to have a high RIN. Superluminescent diodes (SLD""s), on the other hand, tend to have a lower RIN due to their larger bandwidth, but may suffer from limitations in power and lifetime, limiting their utility.
In one approach described in U.S. Pat. No. 5,331,404 and illustrated in FIG. 11, RIN in fiber-optic sensors is reduced by coupling a fraction of the light emitted by a light source 19 into a xe2x80x9cdummyxe2x80x9d fiber 30 having substantially the same length as the fiber 22 of the fiber-optic sensor 5. The output signal detected at the end of the xe2x80x9cdummyxe2x80x9d fiber 30 by detector 34 is then modulated in multiplier 36 by a replica of the signal output of the fiber optic sensor 5 detected by detector 32 and subtracted in subtractor 38 from the output signal of the fiber-optic sensor 5 detected by detector 32 after passing through AC coupled amplifiers 40, 42 with suitable adjustment of the channel gains. In other words, the xe2x80x9cdummyxe2x80x9d fiber 30 in this case operates as an analog delay line to match the time delay experienced by the light traversing the fiber-optic sensor 5. This approach, however, requires a second coil of fiber of approximately the same length as the fiber-optic sensor coil.
In another approach disclosed in U.S. Pat. No. 5,655,035, two fiber-optic sensors can be excited by the same optical source, but with the sensitive axes oriented in diametrically opposed directions. The detected outputs are added, thereby subtracting the RIN, which is common to both channels since it arises in the common source. This approach doubles the entire fiber-optic sensor optical component count (except for the light source), which is expensive and bulky.
It would therefore be desirable to provide a fiber-optic sensor that does not add weight and complexity, while at the same time eliminating or at least reducing the RIN noise component.
The invention is directed to an apparatus and a method for reducing noise, in particular RIN noise, in fiber-optic sensors.
According to one aspect of the invention, a fiber-optic sensor system includes a light source producing a sensor input light, and a fiber-optic sensor receiving the sensor input light and adapted to measure a physical quantity. The fiber-optic sensor produces a sensor output signal which includes the physical quantity and a first noise component, which is detected by a first detector. A second detector detects the sensor input light produced by the light source which includes a second noise component. An electronic processor is coupled to the first and second detectors and includes a delay circuit which applies an electronic time delay to the second noise component, wherein the electronic time delay substantially corresponds to a time delay of the sensor input light traversing the fiber-optic sensor. The electronic processor further includes a multiplier, which multiplies the time-delayed second noise component with the sensor output signal and a subtractor which subtracts the multiplied time-delayed second noise component from the sensor output signal to produce the fiber-optic sensor signal having the reduced noise.
According to another aspect of the invention, a method is provided for producing a measurement signal having reduced noise from a fiber-optic sensor. The method includes measuring a sensor output signal containing the measurement signal and a first noise component and measuring an input light noise component of fiber-optic sensor input light representing a second noise component. The second noise component is electronically time-delayed, wherein the time delay substantially corresponds to a sensor time delay of the sensor input light traversing the fiber-optic sensor. The time-delayed second noise component is then multiplied with the sensor output signal and the multiplied second noise component is subtracted from the sensor output signal to produce the reduced noise measurement signal.
According to yet another aspect of the invention, a method is provided for producing a measurement signal having reduced noise from a fiber-optic sensor. The method includes measuring a sensor output signal containing the measurement signal and a first noise component and measuring an input light noise component of fiber-optic sensor input light representing a second noise component. The second noise component is multiplied with the sensor signal and the sensor output signal and the multiplied second noise component are transformed into the frequency domain. At least one of an amplitude and a phase of the transformed multiplied noise signal is adjusted relative to a respective amplitude and phase of the transformed sensor output signal, so that the relative phase shift corresponds to the sensor time delay. The multiplied second noise component is subtracted from the sensor output signal to produce the reduced noise measurement signal.
According to still another aspect of the invention, an optical sensor system for producing a sensor signal having reduced noise includes a light source producing a sensor input light and an optical sensor which receives the sensor input light and is adapted to measure a physical quantity, with the optical sensor producing a sensor output signal comprising the physical quantity and a first noise component. A first detector detects the sensor output signal, whereas a second detector detects the sensor input light comprising a second noise component. An electronic processor is coupled to the first and second detector and electronically time-delays the second noise component with respect to the sensor output signal, with the electronic time delay substantially corresponding to a time delay of the sensor input light traversing the fiber-optic sensor. The processor subtracts a modulated signal, which is a function of the multiplied time-delayed second noise component, from the sensor output signal to produce the sensor signal having the reduced noise.
Embodiments of the invention may include one or more of the following features. The measured physical quantity may be a rotation of the fiber-optic sensor or a magnetic field which may be produced by an electric current. The second detector which detects the input light noise component, may be coupled to an input section of the fiber-optic sensor, or may be coupled directly to the light source which produces the fiber-optic sensor input light. The electronic processor may include a multi-channel sample-and-hold device and an electronic delay unit which electronically time-shifts the sampled second noise component relative to the sampled sensor output signal by a time period which substantially corresponds to the sensor time delay of the sensor input light traversing said fiber-optic sensor. Alternatively, the processor may include a transform processor, such as a Fourier-transform processor performing discrete Fourier transforms, which transforms the sensor output signal and the modulated noise signal into the frequency domain. A spectral weighting function may be applied to the transformed sensor output signal and the second noise component. The relative amplitude and phase may be determined by comparing the sensor output signal and the second noise component in a spectral region where no measurement signal is produced by the fiber-optic sensor. The relative gain of the sensor output signal and/or the second noise component may be adjusted, for example, by setting the relative gain correction equal to the ratio of the noise powers of the sensor output signal and the second noise component.
Further features and advantages of the present invention will be apparent from the following description of certain embodiments and from the claims.